Solenoids are widely used to convert electrical energy into mechanical movement and, due to their utility, are used in a wide range of applications, from time-critical valves to power locks in automobiles to simple doorbells.
Briefly, a solenoid consists of an electrical wire, typically circularly wrapped to create a coil. A magnetically conductive rod is placed inside the coil. When current passes through the coil, a magnetic field is created, which causes the rod to move relative to the coil. In numerous embodiments, a biasing member such as a spring is used to return the rod to its inactive state when the current ceases to flow through the coil.
A simple example of a solenoid is the traditional doorbell. When the user actuates the doorbell, that action connects the coiled wire to the power source, thereby creating a magnetic field within the coiled wire. This field causes a magnetically conductive plunger to move and typically strike a metal tuning plate, which creates the first “ding” sound typically heard. After the doorbell has been released, a biasing member, such as a spring, returns the plunger to its inactive position. In some doorbells, the plunger strikes a second metal tuning plate, creating a second “dong” sound. These same principles apply in more complex applications, such as valves. In that embodiment, the movement of the plunger typically reveals an opening which is normally obstructed by the plunger, thus opening the valve.
Because of the variety of applications, there are a number of different methods of driving these solenoids. In some cases, cost is the most important factor, while in others, power consumption or speed may be the most important factor.
A number of different embodiments of solenoid driver circuits are commercially available. In fact, several manufacturers produce integrated circuits that perform this function in a single chip. To conserve power, some embodiments regulate the amount of current that is driven through the coiled wire. One common technique used to do this is to vary the voltage supplied to the solenoid, using a technique known as “chopping”, to insure that the current remains constant. The voltage is varied typically in a sawtooth pattern to limit the current to a predetermined value.
In other embodiments, a dual energy level driving system is used. It is a well-known characteristic of solenoids that the energy needed to activate the solenoid and cause movement of the rod is greater than the energy needed to hold the rod in this active state. As a result, some circuits utilize two different levels of current or voltage to drive solenoids; a first, or higher, level needed to “set” the solenoid and a second, or lower, level needed to “hold” the solenoid.
However, in current embodiments, these power saving mechanisms create uncertainty or jitter in the timing of the solenoid. For example, some applications require a precise relationship between the time that the energy is supplied to the coil and the time that the solenoid is activated. By using a simple circuit, which supplies voltage upon the assertion of a specific enabling signal, this predictability can be obtained, however it is not power efficient.
Circuits that control the current by “chopping” the voltage reduce overall power consumption, but are not as predictable with respect to the time between the enabling of power to the solenoid and the activation of that solenoid, due to the variation in the chopped voltage being supplied to the solenoid.
Similarly, circuits that implement two different voltage or current levels experience timing variation as well. Typically, these circuits use the enabling signal to indicate that the “set” voltage should be applied. Using a conventional timing delay mechanism, the level is later switched to the lower “hold” voltage. However, since the higher level is being applied as the circuit is being enabled, a race condition can occur in which the sequence of the enabling of the circuit and the application of the higher voltage can be indeterminate. In some instances, the higher voltage will be present when the circuit is enabled, resulting in a fast turn-on time. In other instances, the higher voltage will not be present when the circuit is enabled, but will be present some time thereafter, resulting is a somewhat slower turn-on time.
In some applications, a capacitor is charged while the solenoid is in the off position. When the solenoid has to be activated, the capacitor's charge is added to the available supply voltage to ensure rapid switching of the solenoid by creating a high peak current. For this method, timing of the solenoid is only predictable when the pauses between activations are long relative to the capacitor charge rate.
In many applications, the small difference in timing caused by these power-saving mechanisms has no effect on the operation of the system in which the solenoid is being used. However, there are applications, such as time-critical valve control in the fabrication of integrated circuits, where it is imperative that there be a predictable time period between the enabling of the circuit and the activation of the solenoid. In these applications, circuits typically do not employ any power saving techniques because of the unacceptable timing jitter that results.
Therefore, it is an object of the present invention to provide a system and method for controlling a solenoid that minimizes the jitter between the enabling of the circuit and the activation of the solenoid, while implementing a dual energy level driving scheme to reduce power consumption.